"Our office has received multiple complaints alleging that the San Antonio police chief violated Senate Bill 4, the Texas law prohibiting sanctuary city policies that the attorney general has authority to enforce," Marc Rylander, communications director for Attorney General Ken Paxton, said in a prepared statement to The Dallas Morning News. "We have begun our investigation and demanded that the San Antonio Police Department preserve all of its records relating to the incident."

Late Wednesday, the attorney general's office sent San Antonio city officials a letter notifying them of the investigation and asking for the records preservation. San Antonio Mayor Ron Nirenberg said at a news conference Thursday that the city will cooperate with any lawful investigation.

On Wednesday, Lt. Gov. Dan Patrick asked Paxton to look into whether San Antonio's police chief violated the state law in handling the case, in which he charged a truck driver under the state's human smuggling law but released the 12 immigrants who appeared to have been smuggled in the truck. Critics say the immigrants should have been held for federal authorities.

"Such action could be in direct violation of the recently passed Senate Bill 4 and threatens the safety of citizens and law enforcement," Patrick wrote in a letter to Paxton. "Should your office receive a citizen complaint as required by Senate Bill 4, I encourage you to act swiftly to ensure San Antonio Police Department is in compliance with the law."

It's unclear whether Patrick's letter spurred the complaints. A spokesman for him did not respond to a request for comment.

Mike Helle, the president of the San Antonio Police Officers Association, told Breitbart Texas that he and some of his officers traveled to Austin on Wednesday to meet with the staffs of Patrick and Gov. Greg Abbott. Helle has criticized how McManus handled the case.

"We finally have a higher authority looking into the chief's conduct," Helle told the website. "That is a good thing."

As of Thursday afternoon, the attorney general's office had received six formal complaints against the city of San Antonio — the first filed under the state's sanctuary cities law. A spokeswoman for the attorney general said the office could not confirm whether all related to the police chief's handling of a human smuggling case.

McManus could not be reached for comment Thursday. In the past, he has said police didn't have the authority to hold the immigrants because they weren't being charged criminally, according to the San Antonio Express-News. He has also said immigration agents were notified of the incident and were not prevented from being involved in the case.

Nirenberg defended his police chief at the news conference Thursday, saying agents from Homeland Security were at the scene of the Dec. 23 incident and given "unfettered access" before they passed on taking the immigrants into custody.

"The attacks on Chief McManus are nothing more than political theater based on a fictitious narrative," Nirenberg said.

The Texas Democratic Party also jumped to McManus' aid, saying the investigation was "un-American" and insulted Texas values.

"Let's be clear about what happened here: a suspected human smuggler was arrested. Twelve human trafficking victims, who went through hell, were placed in the safety and security of Catholic Charities," Democratic Party Chairman Gilberto Hinojosa said in a prepared statement. "Public safety was strengthened because our local law enforcement followed best practices and valued building trust with their community. Now, Texas Republicans want to lock up Chief McManus for doing his job."

"Our elected officials and local law enforcement will not be intimidated by this unprecedented overreach," Hinojosa continued.

The letters from Patrick and the attorney general do not indicate which provisions of the sanctuary cities ban allegedly have been violated.

While the investigation is underway, the sanctuary cities ban remains tied up in legal battles. In August, a federal district judge temporarily blocked significant portions of the law. But an appeals court ruled in September that several provisions of the law could temporarily go into effect — including one requiring jail officials to comply with requests from immigration officials in most cases, and another allowing police to ask about a person's immigration status during a routine stop.

The ruling also allowed the state to start taking complaints against local jurisdictions that violate the law. Before this week, the attorney general's office had not received any official sanctuary city complaints.

The appeals court heard arguments in the case in November but has not ruled on whether the ban can remain in place.